Birds, bugs and climate change
Is erratic weather affecting bird nutrition?
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Early on a cool, smoky June morning researchers Alexis Grinde and Annie Bracey load up with gear and strike out across a grassland south of Moorhead, Minn.
They’re gathering data to better understand how changing weather might be affecting nutrition for aerial insectivores, birds that catch insects in flight.
Many studies have shown declines in bird populations and dwindling insect populations. As a wildlife ecologist at the University of Minnesota Natural Resources Research Institute in Duluth, Grinde wants to better understand the connections between birds and insects.
Wooden nesting boxes are set up on posts across this grassy field to attract tree swallows, the subjects of this study.
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“I’ll walk up check the box, grab an adult, and then we'll put an RFID on it,” said NRRI avian ecologist Annie Bracey. The adult swallow disappears in her hand, only its greenish blue head showing as she clips an identification band on one leg and a small yellow tag on the other.
The yellow tag will trigger a scanner that encircles the hole the swallows use to enter and leave the nest box.
“It’s kind of like a bar code scanner to estimate provisioning rates of the adults, to see how often they're visiting the nest and using that as an index of feeding for their young,” explained Bracey.
This information is key to the question these researchers are trying to understand.
While loss of habitat is a factor in the decline of aerial insectivores, insects are a critical food source and changes in insect populations are thought to significantly contribute to the declining bird numbers.
A nutrition mismatch
The theory is that changing weather patterns are causing a mismatch, between the birds migration and breeding cycles and the availability of insects they depend on for food.
Grinde points to spring weather in recent years with big swings from warm to cold. That can significantly change when insects hatch and if they survive to become food for birds.
“That has huge consequences for what the birds are doing and what their potential productivity for the season is,” said Grinde. “And if we're not making more birds here during the breeding season, there's not going to be a population increase ever."
At least one study has found tree swallows are getting smaller, perhaps in response to the effects of climate change on their diet.
After weighing and measuring the adult bird, Bracey carefully removes a half dozen tiny recently hatched birds from the nest. They are four days old with no feathers, just a few wisps of down on their bare skin.
Working quickly to make sure the nestlings are safely handled and don’t get chilled, the researchers weigh and measure the tiny birds to document their growth and collect fecal samples.
DNA analysis will tell them what insects adult birds are feeding the babies.
Insects are also being collected here throughout the breeding season to understand the relationship between the emergence of insects on the landscape and what the birds are eating.
“Figuring out what's going on with those dynamics, how fast the nestlings grow, how many survive, how does that relate to food availability, it's a huge next step in ecology,” said Grinde.
The research sites range across different landscapes in western Minnesota, from agricultural land to grasslands to a mix of the two types of land use.
The researchers planned to also study eastern bluebirds which will nest near tree swallows, but they haven't found that species in the study area over the last two years. Grinde is not sure why bluebirds are absent, but they are also a species in decline.
Critical questions
There hasn't been much research looking at the interface between changing insect populations and breeding insectivores, said Grinde, but it is critical to understanding the future of these birds.
“They're coming to Minnesota to breed in the month of June. And they're coming here for the insects because there's a ton of food, food is not scarce,” she said.
Plentiful food available at the wrong time in the breeding season could be significant.
“That's a real problem because the moms not going to be able to get enough food to actually lay the eggs. The nestlings aren’t going to get enough food or they're going to be delayed in how much food that they're getting,” said Grinde.
“When you don't have that buffet that you came here for it can get kind of rough and that can definitely affect the energy they have to put into breeding.”
This is the second and final year of data collection for this project funded by the Environment and Natural Resources Trust Fund. Grinde hopes to continue the work because long term monitoring of birds and insects can provide important insights on the effects of a changing climate.
